The present invention relates to an exhaust valve mechanism in an internal combustion engine, comprising at least one exhaust valve in each cylinder, for each cylinder a rocker arm mounted on a rocker arm shaft for operating the exhaust valve, a cam shaft with a cam element for each rocker arm, said cam element cooperating with a cam follower at one end of the rocker arm, a valve play take-up device arranged between an opposite end of the rocker aim and the exhaust valve, said valve play take-up device comprising a piston element received in a cylinder chamber disposed in said opposite rocker arm end, and a hydraulic circuit with valve means for supplying or draining hydraulic oil to and from said cylinder chamber, said valve means comprising a controlled check valve, which at a certain low pressure permits flow of hydraulic oil from the cylinder chamber.
SE-A-468 132 reveals as previously known an exhaust mechanism of the above mentioned type which, together with a special type of cam shaft with extra cam lobes can be used to increase the braking power of the engine. The extra cam lobes are dimensioned in this case so that their lift approximately corresponds to the normal valve play of the valve mechanism. By reducing the valve play to zero by means of the valve play take-up mechanism it is possible to obtain an extra lift during a suitable time interval of the exhaust valve, corresponding to the normal valve play. For example, an extra cam lobe can be so placed relative to the ordinary cam lobe that an extra exhaust valve lift is obtained during the latter portion of the compression stroke, resulting in a portion of the compression work during the compression stroke being lost and not being recoverable during the expansion stroke. The result will be that the braking power of the engine increases.
An engine with such an arrangement for increasing the braking effect will, however, have a somewhat lower efficiency than a corresponding conventional engine. This is because the maximum exhaust valve lift and the latest possible exhaust valve closing at zero valve play is utilized for braking operation and not for drive mode, which means that during drive mode a lower valve lift is obtained when the valve play take-up device is not activated, which also means that the exhaust valves will close earlier than if maximum lift can be used in drive mode.
The purpose of the present invention is to achieve an exhaust valve mechanism of the type described by way of introduction, in which the exhaust valve lift curve can be allowed to be the same for drive mode and braking mode, at least in the vicinity of closing.
This is achieved according to the invention by virtue of the fact that the valve means comprise valve means controlled relative to the rocking movement of the rocker arm said valve means being disposed after a certain rocking movement, to open a communication past the check valve to permit drainage of the cylinder chamber.
The lift curve in the vicinity of closing of the exhaust valves is determined by the available space in the combustion chamber with the piston in the upper dead center position. So that the exhaust valves will not hit the pistons during braking mode, according to the invention a rocker movement controlled bypass valve is used which permits such rapid emptying of the cylinder chamber of the valve play take-up piston that the piston will be in its retracted position when the rocker arm cam follower is lying on the regular cam lobe on its latter portion. This means that the valve closes at the same location on the lift curve during braking mode and during drive mode. After closing of the exhaust valve and subsequent closing of the bypass valve, the valve play take-up piston is again activated by supplying oil under pressure to the piston cylinder chamber via the one-way valve as long as the pressure in the hydraulic circuit maintains the blocking function of the one-way valve. In contrast to the known valve mechanism disclosed in the introduction, the cylinder chamber of the valve play take-up piston is filled and emptied once per cycle in braking mode.
In a preferred embodiment of the valve mechanism according to the invention, the bypass valve is formed by a valve slide, which is rotatably mounted in the rocker arm and which has a gear rim in engagement with a gear section fixed relative to the rocker arm shaft. The slide is made with a groove, which in one position of the slide permits draining of the cylinder chamber past the one-way valve.